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Heat pumps and near surface geothermal energy

Introduction

Heat pumps are an uncomplicated and modern way to regulate the temperature level by using thermal energy from different sources like the ground and ambient air. Find out more about the opportunities this technology offers!

Directly beneath the Earth’s surface, prevailing temperatures are in the region of 8 to 12 degrees Celsius. Using near-surface geothermal energy systems, heat pumps can collect this energy and transfer it for specific purposes, e.g. to warm heating and drinking water or to generate cooling energy. Ambient air and ground water can also be used as heat sources.

In new building projects in particular, various heat pump technologies are becoming increasingly attractive because they provide climate-friendly heat throughout the year. They can also be used for air-conditioning during the summer months. A heat pump is much more efficient than a conventional air-conditioning system. The pump saves the heat that it extracts from the building during the summer underground and then uses this to heat the building in winter.

In Germany, the use of heat pumps is funded subject to the fulfilment of certain criteria. As a result, domestic heat pumps using natural coolants already enjoy a market share of five percent (see German Environment Agency, in German). Heat pumps are installed in 30 percent of new builds (see Federal Statistics Office, in German). As a result, German companies are experienced in the installation and maintenance of all types of heat pump technologies.

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Heat pumps are a key technology in the supply of renewable heat since they also allow the use of heat sources whose temperatures are not high enough for use in heat exchangers. They use the heat from low-temperature heat sources such as the ground, water and air and upgrade this heat to a higher temperature for heating purposes. Modern heat pumps operate very efficiently, even though they require additional energy to raise the temperature. The heating energy produced is significantly greater than the additional energy consumed. The additional energy is usually contributed in the form of electricity or gas. Heat can also be used for this purpose if the heat pump is used as a reverse sorption chiller. Since electricity can also be obtained from renewable sources, heat pumps can also be used in hybrid systems.

If electricity or gas are used, compression heat pumps are necessary since the energy sources then drive a compressor. Different models of heat pumps are available, depending on the medium that releases and receives the heat. Water is usually used as the receiving medium. In water-to-water heat pumps, the heat is collected through heat exchange with ground water. In other cases, the heat is transferred to the air in a ventilation system. An air-to-air system provides heat for the ventilation system through an exchange with the outside air. Other combinations with air, ground and water heat sources are possible, depending on the locally available heat.

Since it is technically easier to provide smaller heating output with low-temperature heat sources, this solution is currently mainly used for space heating in private households or small businesses. In these cases, heat pumps are usually combined with another technology based on oil, gas or biomass in order to cover periods of peak consumption. Larger- scale applications up to an output of 2,500 kW are also possible. If the heat pump is operated in reverse, it can act as a cooling unit and cover a cooling requirement. Depending on the technology, this is either a compression or sorption chiller.

Near-surface geothermal energy (from depths less than 100 m) can be exploited to provide heating or cooling. It is used in combination with a heat pump. Especially during cold weather periods, the near-surface temperature is not sufficient for heating and must therefore be raised by the heat pump.

The low-temperature heat used for the heat pump is provided by collectors installed at 80 –160 cm below the surface, where the soil temperature usually does not fall below 5° C, and varies according to the size of the collectors. Alternatively, a borehole heat exchanger goes deeper into the ground (more than 50 m) and provides heat at a higher temperature, but is also more difficult to install.

Depending on the specific technology used, facilities of different sizes – from small residential units to larger complexes – can be supplied with the required heating or cooling energy. Near-surface geothermal energy installations are primarily used for residential applications since the obtained temperatures are usually not high enough to provide energy for industrial processes.